Unconventional Gas Fracture Logging Method and Apparatus

ABSTRACT

A method for logging slick water hydraulic frac well includes the steps of: Perforating the well bore; Pumping fresh water into the reservoir; Running a high resistivity measurement tool down the hole to measure the resistance of the reservoir at selected intervals along the hole; Logging the resistance measurements; and Comparing the resistance of the reservoir strata with the resistance of the fresh water pumped into the reservoir to map the fracture. It is desirable but not necessary to run a high resistivity tool down the hole prior to the perforation step and measure the resistance of the reservoir to provide base line logging data, whereby the base line logging data may be compared with the water frac logging resistance measurements.

BACKGROUND

1. Field of the Invention

The subject invention is generally related to logging systems for data logging oil and gas wells and is specifically directed to a method and apparatus for logging wells employing slick water hydraulic fracturing.

2. Discussion of the Prior Art.

Hydraulic fracturing, or fracing, is used to initiate production in low-permeability reservoirs and re-stimulate production in older producing wells. In hydraulic fracing, a fluid is injected into a well at pressures so intense that the structure ‘cracks,’ or fractures. Fracing is used both to open up fractures already present in the formation and create new fractures.

Fracture fluid can be oil-based, water-based, acid-based or gel. However, water fracs are the most common and least expensive. Slick water frac jobs are the primary technique used for developing tight-gas reservoirs. In order to retrieve gas at a commercially profitable rate, most tight-gas reservoirs need to be fractured. As part of the frac procedure, propping agents are injected along with the fluid to prop open the new fractures. Typically, a propping agent, or proppant, is a granular substance (sand grains, aluminum pellets, or other material) that is carried in suspension by the fracturing fluid and that serves to keep the cracks open when fracturing fluid is withdrawn after a fracture treatment.

In order to effectively select the right combination and concentrations of frac fluid and propping agents, geologists must know a lot about a reservoir. To create the right approach to a frac job, geologists gather information from well logs about a variety of factors such as porosity, permeability, saturation levels, pressure and temperature gradients. Using this information, geologists run scenarios through 2D or 3D reservoir models to predict the outcomes of various approaches.

Slick-water fracs combine water with a friction-reducing chemical additive which allows the water to be pumped faster into the formation. Slick water fracs work very well in low-permeability reservoirs. In shale formations, brine water is used because the salt content inhibits the formation from swelling. Freshwater is used in other formations where swelling of the clays is not a problem.

The economic viability of field-development projects in low-permeability or “tight” gas sands depends on identifying optimum field well spacing in order to maximize gas recovery with the fewest number of wells. Most wells completed in tight gas sands require some type of hydraulic fracturing to achieve economic production. Wells with longer, more conductive fractures recover more gas over a larger drainage area, thus requiring fewer wells drilled on a larger spacing.

Water fracturing or “water fracs” are less expensive alternatives to the large conventional gel treatments. Although excellent for transporting proppant, these gels often damaged the fracture, usually generated high-net fracturing pressures and were expensive.

Water fracs generate fractures by injecting water with little or no proppant. “Slick-water fracs” employ linear gels or friction reducers to the water. Water fracs can generate similar or sometimes better production responses than large, conventional gel treatments. Microseismic imaging has shown that water fracs can create very long fractures. However, since water is less efficient than gels at carrying proppant, the effective fracture half-lengths may vary significantly, depending on proppant concentration and placement effectiveness.

The use of little or no proppant in a water frac may also result in low fracture conductivities. Fracture conductivity may be either proppant- or asperity-dominated, depending on both proppant and rock mechanical properties. Under asperity-dominated conditions, fracture conductivity depends on the surface roughness or asperities created on the fracture face. High conductivity water fracs can be generated in the absence of proppant only when rock displacement creates ample surface roughness to provide sufficient fracture width. Similar results are derived from low-strength and low-concentration proppants. As a result, under prior art methods, effective fracture conductivities are often difficult to predict when little or no proppant is used.

Hybrid water-frac technologies, which combine the advantages of both conventional gel and water-frac treatments use water to generate fracture width and length while keeping net pressures low. Following creation of fracture geometry, gels with relatively low guar concentrations are used to transport proppant down the fracture. Lower settling rates associated with the gels also allow a more uniform and consistent distribution of proppant placement prior to fracture closure.

SUMMARY OF THE INVENTION

The subject invention is directed to the apparatus and method for measuring the resistivity characteristics of slick water fracs in low-permeability reservoirs making such reservoirs more economically viable than possible with prior art logging and fracing techniques. The bounds of mapping a fracture network is determined between the formation resistivity and the chloride content of the water used in the fracturing job.

The subject invention is specifically directed to a method for logging slick water hydraulic frac well. In the preferred embodiment, the method comprises the steps of: Perforating the well bore; Pumping fresh water into the reservoir; Running a high resistivity measurement tool down the hole to measure the resistance of the reservoir at selected intervals along the hole; Logging the resistance measurements; and Comparing the resistance of the reservoir strata with the resistance of the fresh water pumped into the reservoir to map the fracture. It is desirable but not necessary to run a high resistivity tool down the hole prior to the perforation step and measure the resistance of the reservoir to provide base line logging data, whereby the base line logging data may be compared with the water frac logging resistance measurements.

In the preferred embodiment the high resistivity tool is a cased hole formation resistivity tool such as, by way of example, the CHFR tools offered by Schlumberger. Typically, the method is practiced in a cased, cemented hole.

In the preferred application the resistivity of the fresh water is measured prior to pumping it into the hole. The method may be used in a slick water hydraulic frac application whether or not the fresh water contains a proppant.

Specifically, the subject invention makes possible the use of logging data from slick water fracs in many reservoirs where gel fracing was previously required in order to obtain reliable data.

In the preferred embodiment of the invention, non-conductive fresh water is pumped during the fracture job using know slick water fracing procedures. A resistivity tool is first placed in the casing, prior to slick water fracing, in order to derive a baseline log. Slick water fracing is then performed and resistivity tool is used to generate a post frac log. This will permit mapping of the fracture network created by the slick water frac.

Typical tools will permit mapping from seven (7) to thirty-two (32) feet deep into the formation for a full 360° and will reveal how far the fracture network traveled laterally.

In accordance with the teachings of the invention, it is not necessary to run a baseline log. However the differential readings between the baseline log and the post frac log are very precise and are useful in the mapping process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a typical cased resistivity tool for use in practicing the method of the subject invention.

FIG. 2 is an illustration of a cased hole in a formation which is to be slick water fraced, showing the tool of FIG. 1 positioned in the casing.

FIG. 3 is a flow chart of the steps of the method in accordance with the subject invention.

DETAILED DESCRIPTION

The subject invention employs a deep resistivity tool designed to be run in cased holes. Depending on rock and fluid properties, these tools can measure resistivity 7 to 32 feet deep into the formation (i.e., beyond the casing and wellbore). A typical tool used for this operation is the Schlumberger Cased Hole Formation Resistivity Tool or CHFR tool, as shown in FIG. 1. This tool 10 provides deep-reading resistivity measurements from behind steel casing 12 in a cased hole, see FIG. 2. The tool induces current that travels in the casing 12, where it flows both upward and downward before returning to the surface along a path similar to that employed by open hole laterolog tools. Most of the current remains in the casing, but a very small portion escapes to the formation. Electodes 14, on the tool measure the potential difference created by the leaked current, which is proportional to the formation conductivity. In standard applications, typical formation values are about 10⁹ times the resistivity value of the steel casing. The measurement current escaping to the formation causes a voltage drop in the casing segment.

Because the resistance of the casing is a few tens of microohms and the leaked current is on the order of a few milliampers, the potential difference measured by the CHFR tool is in nanovolts. The electrodes 14 are in direct contact with the casing, making the CHRF tool functional in operations using conductive borehole fluids and will operate in wells with oil, oil-base, or gas in the casing. The abilities to detect and evaluate bypassed hydrocarbons and to track fluid movement in the reservoir are fundamental to improving production and increasing reserves.

In order to obtain the contact between the tool and the casing, the small electrodes 14 on the sonde 16 are designed to scrape through small amounts of casing scale and corrosion to establish good electrical contact. The depth of investigation of the resistivity measurement is between 7 and 32 feet (2 and 10 m), which is more than a magnitude deeper than that of pulsed-neutron saturation measurements.

The present invention is a method which has been developed for using this same technology in slick water frac applications. As shown in FIG. 2, the tool 10 is positioned in the cased hole with the electrodes 14 in contact with the inner wall of the casing 12. In a slick water frac application two readings are typically taken. A base line reading is taken before fracturing by running the CHFR tool, and then, as quickly as possible, a post frac reading is taken permitting mapping of the fracture network created by the fracture job 7 to 32 feet deep into the formation. The process may be run with the baseline reading, but the baseline reading is useful for comparison purposes. The differential readings between a baseline and post frac log will be very precise.

Slick water hydraulic fracture jobs employ fresh water. The source water is usually from a municipal source or fresh water stream, river, or lake. The Chloride content of typical fresh water frac job is 0 to 1000 PPM. This fresh, or low chloride water, is very resistive, that is, it is non conductive to electrical current.

A water with a 1000 ppm chlorides will have a of 100 Ohms or greater. Shale formation resistivity is typically between 10 and 30 Ohms. When this condition exists, the measurement of resistance by the CHFR tool is an excellent mapping technique by tracking the flow of water into the formation fractures.

However, it is desirable to recapture this water and reuse in order to conserve fresh water supplies. As the water is recycled, its chloride content increases. Water re-use for slick water fracture jobs is a big issue. That is, frac flow back water is treated and reused for the next job. Frac flow back water has a higher chloride content than the water originally pumped. Frac jobs utilizing frac flow back water may have chlorides as high as 150,000 PPM or more. The logging method of the present invention is applicable to high chloride content jobs. As the chloride content of the water increases the water becomes more conductive (i.e., less resistive).

A slick water fracture job places water and proppant into the formation via perforations or port collars, i.e., perf clusters. The perforations and port collars are spaced out a pre-determined distance by the completion engineer. Prior to the method of the present invention, how far back into the formation the proppant and fresh water travel was not measurable with any precision. Furthermore, the lateral distance the fracture network travels between perforation clusters (or port collars) was not measurable.

The non-conductive fresh water pumped during the fracture job will show up as high resistive streaks via the CHFR tool. This mapping is 360 degrees and reveals how far the fracture network traveled laterally. For example, this method will determine whether perf cluster #1 fracture network traveled up the hole to perf cluster #2.

Beginning flow back operations prior to running the post frac log is possible but may not be advisable.

The bounds of mapping a fracture network with the CHFR tool will be determined between the formation resistivity and the chloride content of the water used in the fracturing job.

A flow chart showing the preferred procedure for employing the method of the present invention is shown in FIG. 3. The exemplary completion program assumes a shale gas horizontal well has been drilled to total depth, cased, and cemented, as illustrated in FIG. 2. For purposes of discussion, this procedure describes only a single stage job with two perf clusters.

As indicate at block 20, the first step is to rig up the coil tubing eline. Next, as indicated at block 22, make up and test the CHFR tool. The made up CHFR tool is then run in the hole to total depth, as indicated at block 24.

Prior to perforating the reservoir a baseline log is logged up at recommended logging speed for the selected CHFR tool to desired depth, as indicated at block 26. This provides a baseline log from total depth. This step is optional but is desirable. The CHFR is pulled out of the hole at the end of this step.

Next, as indicated at block 28, run in hole with perf guns on coil tubing and peforate perf cluster No. 1 and perf cluster No. 2, in the example fifty feet apart. Once this step is completed, pull out of hole, as indicated at step 30. At this point the pumping vendor, testing equipment and the like are rigged up, see block 32.

Next the slick water is pumped into the hole, see step 34. The resistivity of the water is measured prior to this step. The pumping equipment is rigged down as indicated at block 36.

Next the eline coil tubing is rigged up, see block 38, and the CHFR is run in the hole on the eline coil tubing, block 40. The log re-log intervals are taken at the same intervals as the base line logging step indicated in block 26.

Upon completion of the log re-log function of block 40, the CHFR is pulled out of the hole as indicated at block 42 and the log data may be processed as indicated at block 44.

As described herein, the subject invention provides method for logging slick water hydraulic frac well. In the preferred embodiment, the method comprises the steps of: Perforating the well bore; Pumping fresh water into the reservoir; Running a high resistivity measurement tool down the hole to measure the resistance of the reservoir at selected intervals along the hole; Logging the resistance measurements; and Comparing the resistance of the reservoir strata with the resistance of the fresh water pumped into the reservoir to map the fracture. It is desirable but not necessary to run a high resistivity tool down the hole prior to the perforation step and measure the resistance of the reservoir to provide base line logging data, whereby the base line logging data may be compared with the water frac logging resistance measurements.

In the preferred embodiment the high resistivity tool is a cased hole formation resistivity tool such as, by way of example, the CHFR tools offered by Schlumberger. Typically, the method is practiced in a cased, cemented hole.

In the preferred application the resistivity of the fresh water is measured prior to pumping it into the hole. The method may be used in a slick water hydraulic frac application whether or not the fresh water contains a proppant.

It should be understood that this procedure is intended to be an example only and that the method may be applied in many applications using slick water fracing techniques. While certain embodiments and features of the invention have been shown and described in detail herein, it should be understood that the invention encompasses all modifications and enhancements within the scope of the following claims. 

1. A method for logging slick water hydraulic frac well comprising the steps of: a. Perforating the well bore; b. Pumping fresh water into the reservoir; c. Running a high resistivity measurement tool down the hole to measure the resistance of the reservoir at selected intervals along the hole; d. Logging the resistance measurements; e. Comparing the resistance of the reservoir strata with the resistance of the fresh water pumped into the reservoir to map the fracture.
 2. The method of claim 1, including the additional step of running a high resistivity tool down the hole prior to the perforation step and measuring the resistance of the reservoir to provide base line logging data.
 3. The method of claim 2, wherein the base line logging data is compared with the logging resistance measurements of step e.
 4. The method of claim 1 wherein the high resistivity tool is a cased hole formation resistivity tool.
 5. The method of claim 1, wherein the hole is a cased hole.
 6. The method of claim 1, wherein the resistivity of the fresh water is measured prior to pumping it into the hole.
 7. The method of claim 1, wherein the fresh water contains a proppant. 